In diagnostic imaging using two three-dimensional images that are obtained by imaging the same subject at different times with the same or different imaging apparatuses (modalities), a non-rigid registration technique is attracting attention, where a transformation function that makes the spatial positions of the subject in the two images conform to each other when the images are superposed one on the other is estimated, and one of the images is deformed using the estimated transformation function, thereby registering the two images. In the non-rigid registration technique, control points that divide the image space at given intervals are set, and an image deformation amount of each control point that maximizes an evaluation function, which evaluates similarity between pixel values of the one of the images deformed by displacing the control points and the other of the images, is determined. Then, based on the image deformation amount of each control point, a transformation function is estimated.
D. Mattes et al., “Nonrigid multimodality image registration”, Proceedings of the SPIE, vol. 4322, pp. 1609-1620, 2001 (hereinafter, Non-Patent Document 1) teaches applying the non-rigid registration to images that are obtained by imaging the same subject with different types of modalities, a PET (Positron Emission Tomography) apparatus and a CT (Computed Tomography) apparatus, where an amount of mutual information is used in the evaluation function as a measure of similarity between pixel values of the two types of images.